The invention relates to a control system and method for controlling neutral idle operation of a vehicle. More particularly, the invention relates a system and method that engages neutral idle operation when a brake system provides sufficient brake pressure to maintain a vehicle at a stopped position on a predetermined incline angle.
Automotive vehicles have internal combustion engines that generate torque that is transferred via a transmission to the vehicle wheels. In particular, transmissions such as continuously variable transmissions (CVT""s) generally have a torque converter that is driven by an engine crankshaft. The torque converter is selectively coupled by a clutch (e.g., a forward or reverse clutch) to a variator. The variator transfers torque via final drive gears and a differential to the wheel axles. When the clutch is actuated, the torque converter (and the engine) is torsionally coupled to the variator (and the wheel axles). Alternately, when the clutch is de-actuated, the torque converter (and the engine) is torsionally de-coupled from the variator (and the wheel axles).
Conventional torque converters include an output impeller driven by the input turbine via hydraulic fluid. When a vehicle is stopped and the wheel brakes are actuated, the variator and an output impeller of the torque converter are stopped from the rotating. Thus, the engine crankshaft and turbine are allowed to rotate while being decoupled from the remainder of the drive train. However, during this time, fluid slip between the rotating turbine and the non-rotating impeller reduces engine efficiency and fuel economy.
Known vehicles have utilized neutral idle operation when the vehicle is stopped to increase engine efficiency and fuel economy. During neutral idle operation when the wheel brakes are actuated, a torque converter may be torsionally decoupled from the variator and the wheel axles via a transmission clutch. Thus, fluid slip between the turbine and impeller of the torque converter is reduced since the impeller is allowed to spin at approximately the same speed as the turbine. Further, the reduced fluid slip provides for a reduced load on the engine which increases engine efficiency and fuel economy.
One known neutral idle control system discloses engaging neutral idle operation of a vehicle while maintaining the vehicle at a fixed position i.e., without rollback on a hill. The control system determines an estimated incline of the road based on an acceleration of the vehicle prior to the vehicle stopping. Thereafter, the control system engages a transmission hill-hold brake which locks a gear shifter in the transmission to prevent rollback of the vehicle during neutral idle operation. A problem associated with the known control system is that the system cannot reliably maintain the vehicle on a hill incline during neutral idle operation by using the brake force of the wheel brakes alone. Instead the known control system utilizes an additional hill-hold brake in the transmission which increases transmission component costs, manufacturing costs, and warranty costs. Further, the known control system can only initiate neutral idle operation when the vehicle is on a level or uphill incline. Thus, the known control system does not engage neutral idle operation when the vehicle is stopped on a downhill incline which increases engine loading and decreases fuel economy.
The invention relates to a method and control system for controlling neutral idle operation of a vehicle that overcomes the above-identified problems and disadvantages of known control systems and methods. In particular, the method and control system can initiate neutral idle operation while maintaining a vehicle at a fixed position on either and uphill or downhill incline. Further, the method and control can maintain the position of the vehicle on an incline during neutral idle operation using only the vehicle wheel brakes.
The method and control system may be implemented in a vehicle having an engine coupled to a transmission. The transmission includes a first state torsionally disengaging the engine from a vehicle wheel to initiate neutral idle operation. The transmission further includes a second state torsionally engaging the engine to the wheel to end neutral idle operation.
The method for controlling neutral idle operation of the vehicle includes measuring a brake pressure in a brake control line. The control line communicates fluid to a brake coupled to a wheel of the vehicle. The method further includes determining an incline angle of the vehicle. The method further includes determining a desired brake pressure based on the incline angle. The desired brake pressure represents a brake pressure sufficient for the vehicle brakes to hold the vehicle at a fixed position on the incline angle. Finally, the method includes controlling an operational state of the clutch based on the measured brake pressure and the desired brake pressure while maintaining the vehicle at a predetermined position.
The control system for controlling neutral idle operation in a vehicle includes a pressure sensor generating a signal indicative of a brake pressure in a brake control line that communicates with a wheel of the vehicle. The control system further includes a sensor generating a signal indicative of an incline angle of the vehicle. The control system further includes a controller receiving the pressure signal and the incline angle signal. The controller is configured to determine a desired brake pressure based on the incline angle. The controller is further configured to control an operational state of the clutch based on the brake pressure signal and the desired brake pressure while maintaining the vehicle at a predetermined position.